This invention relates to ink jet printing, and, more specifically, to ink jet printing systems used to imprint packages or cartons with various indicia as the packages are conveyed past an ink jet printhead at a printing station positioned along a conveyor path. Such inkjet printing systems are oftentimes used to imprint shipping information, bar codes, lot numbers and other production or shipping information on overcartons or secondary packaging in a production packaging line or the like. The printhead of such inkjet printing systems is typically supplied with ink from an ink supply remote from the printhead by means of appropriate ink tubes or lines.
Because the printheads are located in close proximity to the cartons (or other objects to be imprinted) as they are conveyed past the printhead if a carton is not properly positioned on the conveyor line, the carton may come into contact with the printhead as the carton is conveyed therepast. In some applications, the cartons are conveyed past the printhead with considerable speed (up to 150 feet/minute or more) and the cartons are heavy. Upon the printhead being hit by one of these heavy cartons being conveyed at such speeds, a considerable impact or shock load is imparted to the printhead. It is known that such impact loads can cause the printhead to de-prime.
It is believed that upon the above-described shock load being imparted to the printhead, a back pressure or shock wave is generated within the ink supply line which travels at extremely high speed through the ink supply line toward the ink reservoir. This shock wave can so reduce the pressure within the ink supply line as to de-prime the printhead. More specifically, and especially with capillary ink feed systems, it is believed that the shock wave may generate back pressures in the ink supply system sufficient to break the meniscus of the ink in the ink orifices of the printhead thus de-priming the printhead. Such de-priming of the printhead is a serious problem.
In the event the printhead de-primes, the printhead will not print until it again is primed with ink. If cartons conveyed past the printhead in a production packaging line are not imprinted, the cartons must be removed from the production line and must either be manually marked or, after the printhead is re-primed, must be positioned on the conveyor line so as to be again conveyed past the printhead for being properly imprinted by the printhead. This, of course, can cause major problems on a production line using such ink jet printing systems.
In addition, it is a time consuming process to re-prime a printhead during which time the packaging line on which the printing system is installed must be shut down. Of course, it is highly undesirable and costly to shut down a production packaging line. In addition, with certain ink jet printing systems, special inks are required to prime the printheads. These special priming inks are expensive and are time consuming to use.
It has long been a goal for such ink jet print systems, and particularly for capillary ink feed systems, to lessen the tendency of the printhead to de-prime. One way of reducing the tendency of the printhead to de-prime has been to incorporate a check valve in the ink supply line between the ink reservoir and the printhead. Upon a back pressure or shock wave being generated in the printhead and traveling back through the ink supply line, and upon this back pressure or shock wave encountering the check valve, the check valve will close thus preventing the shock wave from traveling to the ink supply. However, it has been found that the incorporation of prior art check valves (as hereinafter described in detail), in the ink supply circuit has not abated the tendency of the printhead to de-prime. It is believed that movement of the check valve member from its open to its closed position can sometimes generate a region of low pressure within the ink supply system which can cause a pressure differential of sufficient magnitude to result in de-priming of the printhead.
Still further, the incorporation of a check valve in the ink supply system has other draw backs. First, if the check valve is normally closed, upon initiating flow of ink to the printhead (which is usually in pulses rather than in a steady state flow), the normally closed check valve will require a higher pressure to initially open the check valve (referred to as a cracking pressure). Further, such check valves are susceptible to contamination from particles in the ink such that an accumulation of such contamination particles may adversely affect the operation of such check valve. Still further, the incorporation of such a check valve in the ink supply lines causes a flow restriction that may adversely affect the flow of ink to the printhead and may increase the response time of the ink supply system to the printhead.
As noted, prior art printheads have used check valves in the past. As shown in FIG. 10 of the drawings, a first embodiment of such a prior art check valve is shown which has been used with a capillary ink supply system for an ink jet printhead. This prior art check valve, as indicated in its entirety at 101, has a valve body 103 having an inlet 105 and an outlet 107 with a check valve chamber 109 therewithin. A check valve member, as indicted at 111, is provided in chamber 109 which is movable from a closed position in which the downstream face of the check valve member is in sealing engagement with the downstream face of the chamber 109 surrounding inlet 105 so as to block the backflow of ink from chamber 109 into inlet 105. Upon the check valve member 111 being subjected to normal flow via the inlet 105 from the ink supply to the printhead, the flow will cause the check valve member 111 to shift from its above-described closed position to an open position within chamber 109 in which ink may flow around the periphery of the check valve member 111 and to be discharged from the outlet 107 for flowing to the printhead. In such prior art check valves, the check valve member 111 was typically made of a flexible, resilient elastomer, such as a suitable silicone rubber material or the like, and the check valve member has a diameter somewhat less than the inner diameter of chamber 109 such that the check valve member is free to move within the chamber between its open and its closed positions. As shown, with the check valve member 111 in its open position, the ink is free to enter the chamber 109 on the downstream face of check valve member 111 and to flow around the periphery of the check valve member and to flow to outlet 107.
It will be appreciated that the average flow rate of ink through the above-noted check valves to the printhead is very low (e.g., about 0.5 ml./min.). Moreover, the size of such check valves is small. For example, the diameter of the check valve member 111, as shown in FIG. 8 may only be about 0.110 inches. Referring again to the check valve shown in FIG. 7, upon a shock wave (back pressure pulse) traveling from the printhead to the check valve, the shock wave will travel through the outlet 107 and will enter chamber 109. There, the back pressure or shock wave will act against the entire upstream face of valve member 111 thus causing the member to move axially within chamber 109 to its closed position. However, upon the valve member moving within the chamber from its open to its closed position, the volume of the chamber on the upstream side of the valve member expands greatly and thus generates a low pressure void within the valve chamber. This in turn lowers the pressure within the ink supply line upstream from the check valve and within the printhead. This low pressure may be sufficient to overcome the meniscus force of the ink within the ink orifices of the printhead and thus may result in de-priming of one or more orifices of the printhead. Thus, even with the presence of such check valve in the ink supply system, the check valve did not eliminate the de-priming problem and may even be a cause of printhead de-priming.
In an effort to overcome the shortfalls of the check valve shown in FIG. 10, a second embodiment of a prior art check valve, as shown in FIG. 11, has been used with such ink jet printing systems in an effort to further minimize the tendency of the printhead to de-prime upon the printhead being struck by a carton, as above-described. In this other embodiment of a prior art check valve, the check valve, as indicated in its entirety at 201, has a valve body 203 having an ink inlet 205 and an ink outlet 207 with a check valve chamber 209 therebetween. Similar to valve 101 heretofore described, check valve 201 has an elastomeric check valve member 211 disposed in chamber 209 for blocking back flow from the chamber to inlet 205 when the check valve member 211 is in its closed position. In addition, a part spherical or a conical support 213 is provided at the downstream side of the chamber and the support has an apex 215. Support 213 is of open construction so that ink may flow through the support to the outlet 207. Check valve member 211 is disposed between the downstream face of chamber 209 and support 213 such that the center of the downstream face of the check valve member is engageable by apex 215 of support 213. Check valve member 211 is normally of a flat, planar shape. However, upon installation of check valve member 211 in chamber 209, the check valve member is deformed into a convex configuration, as shown in FIG. 11, in which the outer margins of the downstream face of the check valve member are in sealing contact with the downstream end of chamber 209 so as to block the flow of ink from inlet 205 to outlet 207. Upon a slight pressure differential within chamber 209 so as to cause ink to flow from inlet 205 to outlet 207, the apex 215 is engaged by the check valve member and the outer margins of the check valve member are caused to flex inwardly away from the sides of chamber 209 and the upstream face of the check valve member moves clear of the inlet face of the chamber thereby to enable ink to flow to outlet 207.
Upon a shock wave being generated in the printhead (in the manner above described), the shock wave will enter chamber 209 via outlet 207 and will act against the concave upstream face of valve member 211 facing conical support 213. This causes the valve member to shift toward its closed position and the outer edges of the check valve member move outwardly so as to sealingly engage the walls of chamber 209 and to check the backflow of the ink.
It will be appreciated that the check valves of FIGS. 10 and 11 are not drawn to the same scale. Specifically, check valve 201 shown in FIG. 11 has a considerably larger cross section than check valve 101 shown in FIG. 10. For example, the diameter of check valve member 211 is about three (3) times the diameter of check valve member 111. As a result of this larger size, the check valve member also allows a low pressure zone to be formed within chamber 209 which can result in de-priming of the printhead. Further, check valve 201 is also susceptible to contamination particles interfering with operation of the check valve, and valve 201 still requires a cracking pressure to initiate ink flow.
There has been a long-standing need for a check valve for use in an ink jet ink supply system, which more effectively prevents de-priming of the printhead, which requires less cracking pressure, which is less susceptible to ink contamination particles interfering with operation of the check valve, and which has a faster response time than prior check valves.